Mirage origins
Regarding the origins of the lightning and the mirage III, they both started as concepts for point defence interceptors. It was only with the later IIIE that dassault tried to turn the delta into a multirole aircraft. The one thing that sets the lightning apart from it's contemporaries, and I'm mostly thinking of the sukhoi interceptors, was that it had a wing design that actually allowed it to turn, and that, along with it's abundance of power, meant it could engage in dogfighting, something the sukhois could never do. That was also partly the case of he F106, but the six lacked the lightning acceleration, even if it was faster.
My reference to the FRS2 radar was only because it would probably fit the in the nosecone, and as a suggestion of what could have been done within the limits of the concept. The phantom and later the Tornado F3 killed any possibility of a major lightning upgrade way before that.